The Electrical Behavior of Pd/AlN/Semiconductor Thin Film Hydrogen Sensing Structures
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The Electrical Behavior of Pd/AlN/Semiconductor Thin Film Hydrogen Sensing Structures L. Rimai1, M.H.Rahman2, E.F. McCullen3, L. Zhang1, J.S. Thakur1, R. Naik3, G. M. Newaz4, K.Y.S. Ng2, R.J. Baird1, G.W. Auner1 1
Department of Electrical and Computer Engineering Department of Chemical Engineering and Materials Science 3 Department of Physics and Astronomy 4 Department of Mechanical Engineering Wayne State University, Detroit, MI-48202 2
ABSTRACT The device on Si substrates behaves as an MIS capacitor and the response to hydrogen is given by a shift of the capacitance vs. bias profile along the bias voltage axis, whereas the device on SiC behaves as a rectifying diode and the presence of hydrogen causes a shift of the forward current vs. voltage plot. The relatively large forward current, in both cases, indicates that there is measurable electrical transport across the AlN layer, but at the same temperature the turn on bias is different. Either structure contains two rectifying contacts in series, namely a Schottky contact between Pd and AlN and a heterojunction between AlN and the substrate.
INTRODUCTION Earlier work [1-3] has shown that M/AlN/Semiconductor structures (M is either Pt or Pd), on n-type Si or SiC substrates respond to small (ppm) concentrations of H2 in the surrounding gas flow, selectively in the case of Pd. The device on Si behaves as an MIS capacitor with the AlN playing the role of insulator, whereas the device on SiC behaves as a rectifying diode. The reverse bias dependence of the capacitance of the latter does not exhibit the typical shape of that for a MIS capacitor. However they have similar rectifying behavior (I(V) profiles). Either structure contains two rectifying contacts in series, namely a Schottky contact between Pd and AlN and a heterojunction between AlN and the substrate, as depicted in the last figure of this paper. This figure is drawn for the conventional situation where the thickness of the AlN layer is much larger than the carrier Debye length and that the AlN is n type [4].
EXPERIMENTAL RESULTS The SiC substrate was a 6H wafer oriented 3.5 degrees off the c-axis, with net carrier concentration of 1.5x1024 m-3, whereas the Si had a carrier concentration of 1.5x1022 m-3 (phosphorous doped), at room temperature. Fig.1 shows the current vs. bias characteristics of a SiC based device at various temperatures. Fig. 2 shows the analogous plot for a Si based device. The turn on bias of about 1.5 V is clearly displayed by the lower temperature traces of Fig.1 (a), but the logarithmic display of the same data in Fig. 1 (b) shows a behavior rather more complex than that expected from the simple standard diode equation (1)
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I (V ) = I 0 exp(
− Vb V )(exp( ) − 1) ……………(1) kT nkT
This equation 1 assumes conduction across a simple potential barrier Vb as well as ignores the effect of series resistance. If the total potential drop across the structure occurs only on one side of the barrier, n=1. If this is not the case, n is introduced as an empirical factor, which can
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